Many sleep related respiratory conditions are caused by a blockage or a partial blockage of the respiratory tract. Snoring, for example, is caused by a partial blockage of the respiratory tract during inhalation. As the blockage increases, hypopnea or a reduction of air flow to the lungs occurs. Apnea, or a temporary cessation of breathing, can occur when the airway becomes totally blocked. A patient suffering from apnea may stop breathing for reoccurring intervals of from 10 seconds to two minutes or more for severe cases. The sleep apnea patient may have difficulty functioning in a normal manner during the day because of insufficient sleep caused by the apnea events. In severe cases, the patient also can suffer from problems caused by reduced blood oxygen levels.
One form of treatment for severe snoring, hypopnea, sleep apnea and certain other respiratory conditions involves the application of a pneumatic splint to the patients respiratory tract while the patient sleeps. A sufficiently high continuous positive pressure is applied to the patient's airway to prevent its collapse or blockage. Typically, the applied positive pressure is within the range of from 3 to 20 cm H.sub.2 O. When the CPAP apparatus is initially turned on, the apparatus may immediately apply the prescribed pressure to the patient's airway. However, for improved patient comfort which in turn encourages patient compliance with the physicians prescribed treatment, it is desirable to maintain the applied pressure as low as possible while providing the desired therapeutic treatment. Various techniques have been used to minimize the applied pressure. For example, a more comfortable low pressure may be applied to the patient while the patient falls asleep. Generally, the patient will not suffer from hypopnea or apnea during this time. After sufficient time has elapsed for the patient to fall asleep, a controller gradually increases the applied pressure to the prescribed therapeutic level. Such a control process is sometimes referred to as a "soft start". When the mask is first attached to the patient and the CPAP therapy apparatus is turned on, the pressure controller also may be manually or automatically cycled to apply the full prescribed pressure to the patient for a short time to allow the patient to check and adjust for mask leaks before going into a soft start cycle.
In another known type of CPAP respiratory therapy apparatus, the pressure is automatically increased in increments from an initial low pressure in response to the sensing of snoring, hypopnea and/or apnea events. The pressure also may be gradually decreased over a period of time in the absence of such events. Further, the CPAP therapy apparatus may delay any pressure increase for a time sufficient for the patient to fall asleep or may immediately look for abnormal breathing patterns. Systems of this type often sense snoring and other sounds occurring in the respiratory tract as precursors of apnea, or they may sense the absence of changes in the flow or pressure occurring between inhalation and exhalation during an apnea event, or they may sense the patient's breathing pattern through chest expansions and contractions.
The prior art systems which automatically adjust the pressure respond to specific conditions. Unfortunately, the sensors and the controllers are not capable of distinguishing between different conditions which have similar responses. For example, some systems assume that any cessation of breathing results from apnea and accordingly the pressure is incrementally increased up to the maximum prescribed pressure. If the mask is accidentally knocked off or removed by the patient during sleep or if the mask is moved to a position creating leakage between the mask and the patient's face or if the patient begins open mouth breathing, an automatic pressure controller may assume that the patient has stopped breathing because of an apnea event or slowed down breathing because of hypopnea and may automatically increase the pressure to the maximum prescribed level. Another condition which can result in an automatic pressure increase is exhale mouth puffs. Here the patient inhales through the nose and exhales through the mouth. The prior art controllers cannot detect this condition and consequently may consider the breathing to be shallow and increase the applied pressure. If a patient undergoing CPAP therapy must get up and temporarily remove the mask during the night, it is necessary for the patient to remember to manually restart the CPAP controller when the mask in reattached. If the apparatus is left on while the mask is temporarily removed, the controller will respond as if the patient has experienced an apnea event and will increase the mask pressure to the maximum level. The patient then will be subjected to the maximum pressure when the mask is reattached.